A resistively operating nonvolatile memory cell has at least two different electrical resistances, which can be assigned, for example, to the “0” and “1” states. The memory cell may have a higher or lower electrical resistance as a function of the voltage applied and can be switched between these two resistances.
One of the main aims in the development of modem memory technologies is to increase the integration density, which means reducing the feature size of the memory cells on which the memory devices are based.
The technologies used, such as, for example, DRAM, SRAM, or flash memories, have drawbacks, such as, for example, volatility (DRAM), size (SRAM), or low endurance (number of possible read/write cycles). Hitherto, no technology has been able to satisfy these requirements for different applications.
Ionic solid-state memories are a promising technology for nonvolatile memory cells. These materials may be solid-state electrolytes in either amorphous or polycrystalline form with a grain size in the nanometer range.
For example, certain metals, such as, for example, silver or copper, can be dissolved in chalcogenide glasses. The term “glass” in the broader sense is to be understood in very general terms as meaning a melt that has been supercooled in amorphous form and the atoms of which do not have a continuous long-range order, but rather only a locally limited crystalline arrangement (short-range order) in a three-dimensional, unordered network.
Glasses may be both electrical insulators and electrical semiconductors, depending on which ions are present in the glass and whether the ions which are present are mobile or bonded. The conductivity of the silicon glasses can be obtained, for example, by incorporating ions, such as, for example, sodium, lithium, or silver ions in the glass. If metal ions are dissolved in the glass, the system can be regarded as a “solid-state electrolyte” and the glass alone as a “solid-state ion conductor.”
Chalcogenide glasses can be produced based on compounds of the general formula MmXn, where M is one or more metals from the group consisting of Ge, Sb, Bi, and As, and X is one or more elements selected from the group consisting of S, Se, and Te. The indices m and n do not have to be integer numbers, since metals can adopt a number of oxidation states which are present simultaneously.
The chalcogenide glasses are generally semiconducting. Dissolving the metal ions in the chalcogenide glasses produces a solid solution of the relevant ions in the glass. Silver ions can be dissolved, for example, by deposition of an Ag film on a chalcogenide glass and subsequent irradiation. The irradiation of a sufficiently thick film of Ag on Ge3Se7 produces, for example, a material of formula Ag0.33Ge0.20Se0.47. Accordingly, the solutions can be formed by the photo-dissolution of silver in, for example, As2S3, AsS2, GeSe2.
An approach for fabrication of resistive nonvolatile memory cells is based on the use of the solid solutions in chalcogenide glasses as active (switching) material for nonvolatile memory cells. A memory cell of this type has a layer of a chalcogenide glass, in which metal ions of the material from which one of the electrodes is formed are dissolved, arranged between a first electrode and a second electrode.
Chalcogenide glass memory cells are based on an electrochemical redox process, in which metal ions of one electrode are able to reversibly diffuse in and out of the solid-state electrolyte material and thereby to form and remove a low-resistance path. More specifically, the material of chalcogenide glasses is arranged between two electrodes. One electrode is an inert electrode and the other electrode is a reactive electrode.
The ions of the reactive electrode are soluble in the chalcogenide glass.
Hirose et al., Journal of Applied Physics, Vol. 47, No. 6, 1976, pp. 2767 to 2772, “Polarity-dependent memory switching and behavior of Ag dendrite in Ag-photodoped amorphous As2S3 films,” describe an arrangement with an inert electrode of molybdenum or gold, a second electrode of silver, and a layer of a chalcogenide glass of As2S3 photodoped with Ag+ ions arranged between the two electrodes. Application of a positive voltage, which is above the minimum threshold voltage, to the Ag electrode oxidizes the electrode, causes the Ag+ ions to be driven into the chalcogenide glass and reduced again at the inert electrode, which leads to metallic deposits in the form of a conductive Ag path (dendrites) between the first electrode and the second electrode. This lowers the electrical resistance of the arrangement. In this state, the electrical resistance of the solid-state electrolyte is reduced significantly, for example, by several orders of magnitude, compared to the state without a metallic current path, thereby defining the ON state of the memory cell. If a voltage of the opposite polarity is applied to the two electrodes, the metallic deposits and the current path are removed again. As a result, the two electrodes are no longer continuously electrically connected to one another, thereby defining the OFF state of the memory cell, since, in this state, the memory cell has a higher resistance than in the ON state.
Therefore, the general mechanism can be explained as the reactive electrode with the solid-state electrolyte forms a redox system in which a redox reaction takes place above a defined threshold voltage (Vth). The redox reaction can take place in one reaction direction or the other depending on the polarity of the voltage applied to the two electrodes, although this voltage must be higher than the threshold voltage. Depending on the voltage applied, the reactive electrode is oxidized and the metal ions of the reactive electrode diffuse into the chalcogenide glass and are reduced at the inert electrode. If metal ions are continuously released into the solid-state electrolyte, the number and size of the metallic deposits increase until ultimately a metallic current path which bridges the two electrodes is formed (ON state). If the polarity of the voltage is reversed, metal ions diffuse out of the chalcogenide glass and are reduced at the reactive electrode, which leads to the metallic deposits on the inert electrode being broken down. This process is continued, under the influence of the applied voltage, until the metallic deposits which form the electrical path are completely broken down (OFF state). The electrical resistance of the OFF state is two to six orders of magnitude greater than the resistance of the ON state.
An element based on the mechanism described above is also known as a programmable memorization cell (PMC). Another approach is doping the chalcogenide glass with metal atoms, resulting in the formation of conducting islands which, by percolative bridging in a random network, define the ON state.
Hitherto, there has not been either integrated demonstrators nor products for this memory technology. Implementation of individual switching elements based on chalcogenide glasses, such as As2S3, GeSe, or GeS, and WOx, is known. M. N. Kozicki et al., “Superlattices and Microstructures,” Vol. 27, No. 5/6, 2000, pp. 485 to 488, M. N. Kozicki et al., Electrochemical Society Proceedings, Vol. 99-13, 1999, pp. 298 to 309, “Applications of Programmable Resistance Changes in Metal-Doped Chalcogenides,” and M. N. Kozicki et al., 2002, “Can Solid State Electrochemistry Eliminate the Memory Scaling Quandary?” The aforementioned publications propose depositing solid-state electrolyte in a via hole vertically etched in a conventional inter-dielectric (hole between two metallization levels of a semiconductor element). Then, the material of the reactive electrode is deposited and patterned, for example, by a suitable etching process or by chemical mechanical polishing (CMP). This is followed by a process which forces the material of the reactive electrode into the solid-state electrolyte, in order, through UV irradiation, to generate background doping of the solid-state electrolyte with the metal of the reactive electrode.
However, use of chalcogenide materials, as known, conceals drawbacks, such as the limited thermal stability of the chalcogenide glasses requiring special measures for the back-end integration of a fully integrated memory. For example, Se-rich, GeSe has a phase transition of just 212° C., which causes serious drawbacks, in particular, with a view to processing in the back-end region (Gokhale et al., Bull. Alloy Phase Diagrams 11 (3), 1990). The introduction of new materials based on chalcogenide glasses requires a high level of outlay and possibly additional tools in order to prevent contamination during CMOS fabrication. Moreover, some chalcogenide glasses are relatively toxic materials, which require additional precautions for safe operation during production.
Memory cell based on solid-state electrolytes without chalcogenide glasses, and a method for fabricating resistive nonvolatile memory cells, in which the cells are based on solid-state ion electrolytes without using the chalcogenide glasses, are desirable.